1. Field of the Invention
This invention relates to improving the operation of nitrogen oxide traps employed in the exhaust system of an internal lean burn combustion engine by adsorbing sulfur oxides.
2. Background Art
Environmental concerns have required automobile manufacturers to devise strategies for decreasing emission of unwanted combustion byproducts from a vehicles exhaust system. The three unwanted byproducts are carbon monoxide, nitrogen oxides, and hydrocarbons. The nitrogen oxides include both NO and NO2. Three-way catalytic converters (TWC) are used to minimize emission of these byproducts.
When the engine is operated in a stoichiometric or slightly rich air/fuel ratio, i.e., between about 14.7 and 14.4, catalysts containing platinum, palladium, and rhodium or palladium and rhodium are able to efficiently convert all three gases simultaneously. It is desirable to operate an engine under lean burn conditions where the air/fuel ratio is greater that 14.7, generally between 19 and 27 to realize a benefit of fuel economy and reduced CO2 emission. Such three-way catalysts are able to convert carbon monoxide and hydrocarbons but are not efficient in reduction of nitrogen oxides (NOx) during lean-burn operation. The situation is compounded by recent EPA guidelines that require even lower emissions of nitrogen oxides. Such stringent requirements regarding nitrogen oxide have made it desirable to include an additional nitrogen oxide trap in addition to the standard TWC.
The decomposition of NO and NO2 into N2 and O2 is thermodynamically favorable but kinetically inhibited. Therefore, NOx traps will typically contain a catalyst such as platinum. Such traps are believed to first oxidize adsorbed NO to NO2 wherein the NO2 subsequently forms a nitrate with another component of the trap such as barium. Subsequently, during regeneration periods when the engine is operated in an stoichiometric or fuel rich condition, these nitrates are thermodynamically unstable resulting in the release of various NOx species. These released NOx catalytically react with reducing species in the exhaust such as hydrocarbons, CO and H2 over the catalyst to form N2.
However, a serious drawback in the operation of NOx traps is that they are readily poisoned by sulfur contained in the vehicle exhaust gas. Most vehicle fuels contain sulfur which is subsequently converted to various sulfur oxides such as SO2 when the fuel is combusted. These sulfur oxides react with the NOx trap materials to form sulfates. Sulfates are not as readily converted back to the sorption materials during regeneration as are the nitrates. Therefore, the typical NOx trap is strongly deactivated over time by the sulfur compounds contained in the exhaust.